Well, or borehole logging involves making an analysis of the conditions of a well and surrounding geological formations. It may be performed by lowering instruments into the well on a wireline that can take measurements at different points in the well.
Acoustic instruments are used for obtaining information using ultrasonic waves. They include a source of ultrasonic waves and a sensor to measure a response to the produced ultrasonic wave.
During completion of a well, the casing may be cemented. Cementing is used to seal and protect the well, and to create a well barrier to prevent fluid migration. Cementing involves deploying a cement slurry and allowing it to fill a space between a well casing and the wellbore. The required physical properties of the cement may differ depending on the nature of the well, and so the correct type of cement must be prepared. Additives include accelerators, which shorten the setting time required for the cement, as well as retarders, make the cement setting time longer. The density of the cement may be controlled using different additives, and additives may also be used to affect the compressive strength of the cement.
In many territories, it is a legal requirement to cement a casing up to a certain height. As the volume between the well casing and the borehole wall may not be precisely known, pumping a certain volume of cement into the annulus may not give cement up to the required height. There is a requirement for a technique that can accurately determine the height of the cement even when it is located behind the well casing.
A further issue with cementing is mixing of the cement with other fluids. Referring to FIG. 1, when the borehole has been drilled it is full of drilling fluid, termed mud 1. A casing 2 is introduced into the borehole which typically has a steel wall. Cement slurry 3 is introduced into the annulus from the bottom of the borehole between the casing 2 and the sides of the borehole. This displaces the mud 1. Spacer fluid 4 is disposed between the cement slurry 3 and the mud 1. Spacer fluid 4 is a viscous fluid used to aid in displacement of the mud 1 by the cement slurry 3 and to prevent mixing of the cement 3 and the mud 1. When the fluids exit the casing and enter the annulus of the well, there is no physical separation of the fluids, other than their rheological properties, and mixing of the fluids is possible.
It is necessary to ensure that no mixing has occurred between the mud 1, the spacer 4 and the cement slurry 3. Any mixing between the mud 1 and the cement slurry 3 in particular will be detrimental to the final properties of the cement.
There are several techniques for monitoring the properties of cement. For example, US 2008/0236814 describes using MEMs sensors which communicated with an interrogator tool to monitor various properties of the cement. However, this technique is expensive. A more common technique to monitor the properties of the cement 3 is to use an ultrasonic acoustic instrument 5, as shown in FIG. 1. By sending ultrasonic signals from the instrument 5 and interpreting responses, information about cement, such as the degree of mixing, the degree of hardening, casing dimensions and so on can be obtained. By monitoring responses, the instrument 5 can be used to determine the properties of a fluid, and in particular whether the fluid is a mixture of mud 1 and cement slurry 3.
Existing acoustic techniques are limited in their reliability, resolution and range. For example, acoustic logs are generally effective only through a single wall layer. Where a casing has more than one wall, the results of acoustic techniques may give no useful information about the nature of the fluid behind the casing wall 2. In order to log behind a second wall, the first wall needs to be removed as the acoustic waves would otherwise be reflected by the first wall. In many circumstances this is impossible, and even in situations where it can be done it is time consuming and expensive.
The operator therefore has limited knowledge about the height of the cement behind the casing wall, and whether or not any mixing has occurred between the cement slurry 3 and the mud 1, and so does not know whether or not the cementing operation has been successful.